Dad's age not a big player in child's autism risk

NEW YORK (Reuters Health) - Studies that have suggested that older men are more likely to father autistic children have seriously overstated the risk, conclude the authors of a large new analysis of parental age and autism risk.

During the past couple of decades, rates of autism skyrocketed, while fatherhood after 40 also became more common, Dr. Peter S. Bearman of Columbia University in New York City, who helped conduct the new study, noted in an interview with told Reuters Health.

In the prior studies, he explained, researchers looked at the past couple of decades as a single unit, rather than analyzing the relationship on a year-by-year basis.

"If you pool data over a long period of time," Bearman said, "it'll look like there's a relationship between older dads and autism that might not be there."

Some studies have found an increased risk of autism in children born to older dads, but others have not. The reported risk varies widely, with one study finding children born to fathers over 40 at six-fold greater risk of autism compared to those with younger dads.

To better understand the relationship, Bearman and his team looked at records for all children born in California between 1992 and 2000, nearly 5 million in total. According to a report in the American Journal of Public Health, they found 18,731 children who had been diagnosed with autism.

Over the course of the study, the average age of both mothers and fathers rose, while the percentage of moms and dads over 40 also increased.

According to the data, both mothers and fathers over 40 had an increased risk of having a child with autism, but the risk varied by birth year. For older mothers, it ranged from a 1.27-fold increased risk for kids born in 1995, to a 1.84-fold greater risk for children born in 1993. Among men, increased risk ranged from 1.29 for kids born in 1992, to 1.71 to for those born in 1995.

The variation in risk may have something to do with the fact that the population of people with autism has changed over the years, as more and more children are diagnosed with the disease, Bearman and his colleagues suggest.

For example, 40.3 percent of children with autism born in 1992 were mentally retarded, compared to 22.7 percent of autistic children born in 2000.

One problem with teasing out the effects of maternal versus paternal age, the researcher noted, is that older women tend to marry older men, and vice-versa. To account for this, Bearman and his colleagues analyzed paternal age and autism risk independently of maternal age. When they did this, the risk associated with being an older dad disappeared.

However, when the researchers looked at the influence of mothers' age independently of paternal age, the risk remained.

Even then, however, the increased risk of having an autistic child for mothers over 40 was small, at around 3 percent to 4 percent, the investigators found.

The statistical fluke they uncovered, known as the "reversal paradox," might be producing misleading estimates of other types of risk, Bearman said.

"It would be great if other researchers thought more about the consequences of pooling data and worked more explicitly with temporarily sensitive data," he said. "You'd like to do the analysis in ways that are providing the best information for people."